1. The Field of the Invention
The invention generally relates to lasers. More specifically, the invention relates to Vertical Cavity Surface Emitting Lasers (VCSELs).
2. Description of the Related Art
Lasers are commonly used in many modern components. One use that has recently become more common is the use of lasers in data networks. Lasers are used in many fiber optic communication systems to transmit digital data on a network. In one exemplary configuration, a laser may be modulated by digital data to produce an optical signal, including periods of light and dark output that represents a binary data stream. In actual practice, the lasers output a high optical output representing binary highs and a lower power optical output representing binary lows. To obtain quick reaction time, the laser is constantly on, but varies from a high optical output to a lower optical output.
Optical networks have various advantages over other types of networks such as copper wire based networks. For example, many existing copper wire networks operate at near maximum possible data transmission rates and at near maximum possible distances for copper wire technology. On the other hand, many existing optical networks exceed, both in data transmission rate and distance, the maximums that are possible for copper wire networks. That is, optical networks are able to reliably transmit data at higher rates over further distances than is possible with copper wire networks.
One type of laser that is used in optical data transmission is a Vertical Cavity Surface Emitting Laser (VCSEL). As its name implies, a VCSEL has a laser cavity that is sandwiched between and defined by two mirror stacks. A VCSEL is typically constructed on a semiconductor wafer such as Gallium Arsenide (GaAs). The VCSEL includes a bottom mirror constructed on the semiconductor wafer. Typically, the bottom mirror includes a number of alternating high and low index of refraction layers. As light passes from a layer of one index of refraction to another, a portion of the light is reflected. By using a sufficient number of alternating layers, a high percentage of light can be reflected by the mirror.
An active region that includes a number of quantum wells is formed on the bottom mirror. The active region forms a PN junction sandwiched between the bottom mirror and a top mirror, which are of opposite conductivity type (i.e. a p-type mirror and an n-type mirror). Free carriers in the form of holes and electrons are injected into the quantum wells when the PN junction is forward biased by an electrical current. At a sufficiently high bias current the injected minority carriers form a population inversion in the quantum wells that produces optical gain. Optical gain occurs when photons in the active region stimulate the transition of electrons from the conduction band to the valance band which produces additional photons in phase with the stimulating photons. When the optical gain is equal to the loss from the two mirrors and the cavity, laser oscillation occurs. The free carrier electrons in the conduction band quantum well are stimulated by photons to recombine with free carrier holes in the valence band quantum well. This process results in the stimulated emission of in phase photons, i.e. coherent light.
The active region may also include an oxide aperture formed using one or more oxide layers formed in the top and/or bottom mirrors near the active layer. The oxide aperture serves both to form an optical cavity and to direct the bias current through the central region of the cavity that is formed.
A top mirror is formed on the active region. The top mirror is similar to the bottom mirror in that it generally comprises a number of layers that alternate between a high index of refraction and a lower index of refraction. Generally, the top mirror has fewer mirror periods of alternating high index and low index of refraction layers, to enhance light emission from the top of the VCSEL.
Illustratively, the laser functions when a current is passed through the PN junction to inject free carriers into the active region. Recombination of the electrons from the conduction band quantum wells to the valence band quantum wells results in photons that begin to travel in the laser cavity defined by the mirrors. The mirrors reflect the photons back and forth. When the bias current is sufficient to produce a population inversion between the quantum well states at the wavelength supported by the cavity, optical gain is produced in the quantum wells. When the optical gain is equal to the cavity loss laser oscillation occurs and the laser is said to be at threshold bias and the VCSEL begins to ‘lase’ as the optically coherent photons are emitted from the top of the VCSEL.
The VCSEL is generally formed as a semiconductor diode. A diode is formed from a pn junction that includes a p-type material and an n-type material. In this example, p-type materials are semiconductor materials, such as Gallium Arsenide (GaAs) doped with a material such as carbon that causes free holes, or positive charge carriers to be formed in the semiconductor material. N-type materials are semiconductor materials such as GaAs doped with a material such as silicon to cause free electrons, or negative charge carriers, to be formed in the semiconductor material. Generally, the top mirror is doped with p-type dopants where the bottom mirror is doped with n-type dopants to allow for current flow to inject minority carrier electrons and holes into the active region.
Doping the mirrors results in various difficulties in lasers designed to produce longer wavelengths. For example, as wavelength of the emitted light increases, free carrier absorption also increases in the doped mirrors. Free carrier absorption is the absorption of photons by carriers (i.e. electrons and holes) in semiconductors which occurs other than across the band gap. Specifically, free carrier absorption occurs at other non-photon producing bands other than between the valence and the conduction bands.
In particular, holes cause free carrier absorption. Trap related absorption is also prevalent in n-type AlGaAs. This added mirror loss requires higher optical gain in the quantum wells to achieve threshold bias. To achieve higher gain a higher current (i.e. threshold current) must be passed through the VCSEL to cause the VCSEL to lase. The higher mirror loss also lowers the efficiency of the VCSEL. This increases the internal heating and limits the amount of power that a VCSEL can output.
Further, the restriction on doping level caused by absorption in the top mirror decreases the electrical conductivity of the VCSEL which causes resistive heating in the VCSEL which limits power output, and degrades reliability.
Additionally, ramps of material composition at the boundaries between layers in the mirrors degrade thermal impedance and reflectivity. If the VCSEL is not able to conduct heat away from the active region, the operating temperature of the VCSEL may rise. If the mirror layers have a degraded reflectivity, additional layers may need to be used resulting in increased impedance and further increased heating of the VCSEL. Excessive heating can damage the VCSEL or shorten the useful life of the VCSEL or degrade its performance.
In summary, various issues relate to VCSEL technologies, including free carrier absorption, series resistance, reliability, manufacturability, thermal resistance, polarization stability, single mode operation, optical isolation from external reflections and high temperature performance.
While the current designs have been acceptable for shorter wavelength VCSELs such as VCSELs emitting 850 nanometer (nm) wavelength light, longer wavelength VCSELs have been more difficult to achieve. For example, in long wavelength VCSELs, absorption coefficients are larger. Additionally, due to the longer wavelengths and resulting increase in VCSEL layer size, increased absorption occurs. Specifically, VCSELs have layers that are optimized based on the wavelength at which the VCSEL is designed to lase. A quarter wavelength section in a 1310 nm VCSEL is longer than a quarter wavelength section in an 850 nm VCSEL. Thus, a 1310 nm VCSEL has more material where free carriers can be absorbed than an 850 nm VCSEL.
A 1310 nm VCSEL would be useful in telecommunication applications. The market entry point of lasers used in 10 Gigabit Ethernet applications is 1310 nm. However, due to the thermal and optical characteristics and other difficulties described above of currently designed VCSELs, 1310 nm VCSELs have not currently been feasible.